The present invention relates generally to offshore oil and gas drilling, completion, and maintenance operations. More particularly, the present invention relates to apparatus for minimizing the effects of vessel heave during stabbing and landing operations.
In the drilling, completion, and maintenance of many offshore oil and gas wells, it is necessary to stab tubing strings into boreholes and land large equipment packages onto the seabed from a floating vessel These operations often require precise placement of the equipment. Wind, wave, and current induced forces cause relative motion between the floating vessel and the seabed. This relative motion in the vertical direction is termed "heave." Control over movement of the package is needed to minimize the risk of misplacement or damage to the equipment due to heave.
Critical operations such as, for example, landing a Christmas tree, replacing an equipment skid or manifold, or reentering a borehole with a drill bit and drill string require a great deal of precision and control and, therefore, a minimum of relative motion, or heave, between the equipment package and the seabed. Operations may be delayed until the weather and seas are calm. However, a more practical solution is to use heave compensation to avoid loss of rig time.
Heave compensators, also called drill string compensators, are currently utilized in drilling operations to maintain a relatively constant weight on the drill bit and drill string by compensating for the relative vertical movement of a floating drilling vessel with respect to the earth due to heave. Most heave compensators are either integral with the crown block or attached to the traveling block. They bias the drill string with respect to the heaving vessel in order to keep a relatively constant weight on the bit, and thereby maintain the drill string reasonably stationary with respect to the earth. Examples of heave compensation apparatus are disclosed and described in U.S. Pat. Nos. 3,163,005, 3,804,183, 3,834,672, RE 29,564, and RE 29,565.
Heave compensators generally comprise hydraulic and pneumatic systems which adjust the relative elevation of the tubing string with respect to the floating vessel based on the tension in the tubing string. As the weight applied to the tubing string varies due to vertical movement of the floating vessel, the heave compensator reacts to either raise or lower the tubing string in the direction opposite the movement of the vessel. This tends to maintain the desired tension in the tubing string and the relative position of the tubing string with respect to the earth even though the vessel is heaving.
While the heave compensators described above are generally capable of sufficiently compensating for the effects of heave on a drill string in most operations, they still allow some degree of uncompensated relative movement between the drill and the earth. These apparatus are not entirely effective in stabbing and landing operations. Such operations would benefit from a near elimination of the effects of vessel heave on the tubing string and great precision and control over movement of the tubing string in general In addition, the heave compensation apparatus described above are also not entirely effective in operations in which the weight of the drill string is minimal.
Another approach to heave compensation has been used in offshore wireline well logging. There, a tensioned line connected to the marine riser runs over a sheave connected to the vessel's heave compensator and connects to a fixed point in the vessel. This makes the lower end of the heave compensator a relatively vertically stationary point with respect to the marine riser, because relative motion between the vessel and the lower end of the heave compensator is accommodated by the heave compensator. Logging operations are conducted using a sheave connected to the lower end of the heave compensator. This approach still permits some relative movement between the vessel and the earth.
The present invention overcomes the deficiencies in the prior art by providing an apparatus and system which allow the stabbing and landing of equipment packages onto the seabed while nullifying the effects of the vertical motion of the floating vessel due to heave on the tubing string.